(CNN) --The voices inside Charles Singleton's head varied, in volume and number, regardless of whether he had taken medication for his schizophrenia. Inside his Arkansas cell, he said he could often hear voices that speak of killing him.

Singleton was executed by lethal injection Tuesday night at the Cummins prison unit in Varner, about 70 miles south of Little Rock. (Full story)

Singleton's attorney said his 44-year-old client welcomed his execution because he was tired of living with mental illness.

Singleton understood that he would be put to death and why -- the current legal standards to qualify for execution, said his attorney, Jeff Rosenzweig.

Singleton, however, was rational only when he was on medication. It was that fact, as well as an 18-year-old Supreme Court ruling barring executing the insane, that his attorney, some members of the legal and medical communities and death penalty critics pointed to in opposing Singleton's execution.

"If [Singleton] is artificially made to be competent, then the situation is an oxymoron," said Ronald Tabak, a New York-based attorney who has represented clients in death penalty cases.

But the prosecutor in the Singleton case claimed the defendant was sane at the time of the crime, and therefore unaffected by the Supreme Court ruling.

"I do not feel he is being medicated in order to put him to death," said John Frank Gibson, who hasn't dealt with the Singleton case in recent years. "He's being medicated to ... keep him healthy, to control him."

Stay of execution lifted

Singleton was 19 when he stabbed Mary Lou York to death while robbing a small grocery store in Hamburg, Arkansas. She identified him before she died. In 1979 he was convicted and sentenced to death.

A prison psychiatrist in 1997 diagnosed Singleton as suffering from paranoid schizophrenia. That same year, a prison medication review panel ordered Singleton to take antipsychotic drugs after finding he posed a danger to himself and to others.

After the medication took effect, Singleton's psychotic symptoms abated and Arkansas made plans to execute him.

Singleton's attorneys filed a lawsuit arguing the state could not constitutionally restore his client's mental competency through the use of forced medication and then execute him.

In October 2001, a panel of the 8th U.S. Circuit Court of Appeals ruled that Singleton be sentenced to life in prison without the possibility of parole.

The state appealed, and last February a sharply divided full 8th Circuit Court lifted a stay of execution for Singleton.

The court said at the time that because Singleton was voluntarily taking his medication and because Arkansas had an interest in having sane inmates, the side effect of sanity should not affect Singleton's sentence.

In October, the Supreme Court declined without comment to hear the Singleton case.

Most Americans have consistently supported the death penalty since it was reinstated in the 1970s. But polls show that the issue of mental illness sharply affects public opinion.

The son of the victim in the Singleton case says the insanity question is just a ploy.

"I don't believe it," Charles York said. "It's just something they use to prolong things to keep it in the court system."